The rarely told story of the Japanese WWII floating bomb campaign

President Franklin Roosevelt declared December 7, 1941 "a date which will live in infamy" after the Japanese leveled a devastating attack on Pearl Harbor, but while it is by far the best known, that day was not the only time a Japanese bomb took American lives on U.S. soil.

On May 5, 1945, a pregnant Sunday school teacher and five children from a small Oregon town called Bly were killed by a Japanese-built bomb that had floated across the ocean on a balloon.

The bombing campaign has been covered in recent years on film, radio and by historian Ross Coen in his book "Fu-go" but never made major headlines at the time. As Coen notes, only a handful of Americans had any clue about the threat in 1945, even though the Japanese had quietly launched thousands of them.

The bombs were attached to paper-thin balloons propelled by the jet stream from Japan all the way to North America. Hundreds fell in various spots stretching from Alaska to Arizona, with the vast majority never causing any significant damage.

By the time the U.S. government caught wind of the campaign, they immediately sought to censor the news, professor Mike Sweeney explained to RadioLab, worried that Americans might panic. After a handful of findings inspired a few scattered reports from Newsweek and Time magazines, the U.S. Office of Censorship issued a press "blackout" -- insisting that any news of the bombs had to be approved by the Army.

That is why Mrs. Elsie Mitchell and five children had no reason to be concerned when they discovered one of the fallen undetonated bombs near Gearhart Mountain in Oregon. Although their exact reactions remain unknown, the bomb soon went off, killing all five kids and Mitchell. According to the History Channel, they were the only known American civilians killed in the continental United States during World War II.

To learn more about the Fu-go bombing campaign, check out RadioLab's coverage.